Experiment TC 31

Cooperative Practical Training of the

Department of Biochemical and Chemical Engineering

This experiment is part of the experimental courses of Technical Chemistry A

Experiment TC 31

Liquid-Liquid-Extraction Contents:

• Visualization of ternary mixtures in triangular diagrams • Liquid-Liquid-Equilibrium of acetic acid / n-butyl-acetate / water

• Binodal-curve as solubility limit • Tie lines as connection line between to phases in equilibrium • Auxiliary line for the graphical interpolation of equilibrium data

• Experimental verification of some equilibrium data • Balancing of a four-stage countercurrent extraction by stage-construction

in a triangular diagram and in an equilibrium diagram • Experimental performing of the four-stage countercurrent extraction in a continuous

mixer-settler laboratory setup • Comparison of theoretical date with the experimental data

• Determination of the stage-efficiency within the laboratory setup

University of Dortmund Cooperative Practical Training, Experimental Section TCA Sign: Seuster/Jörissen Department of Biochemical TC 31 Page: 2 and Chemical Engineering Liquid- Liquid-Extraction Date: August 2005

For personal Use only!

Index 1. Introduction .............................................................................................................................................3

2. Fundamentals...........................................................................................................................................4

2.1 The Liquid-Liquid-Extraction .................................................................................................................4 2.2 The triangular diagram and its properties................................................................................................5 2.3 Binodal curves and tie lines.....................................................................................................................6 2.4 Procedures for measurement and interpoation and model-equations for liquid-liquid-equilibria ...........8 2.5 The equilibrium diagram .........................................................................................................................9 2.6 Extraction processes ..............................................................................................................................10 2.7 The representation of a countercurrent extraction in a triangular diagram............................................12 2.8 Stage construction with different starting conditions ............................................................................13 2.9 Transmission of the stage construction into the equilibrium diagram...................................................14 2.10 The practical extraction-stage and the effiecency ................................................................................15

3. Experiment description.........................................................................................................................16

3.1 Physical properties.................................................................................................................................16 3.2 Methods for the estimation of liquid-liquid phase-equilibria ................................................................17 3.3 Laboratory plant for the contiuous four Stage countercurrent extraction..............................................18

4. Task formulation and experimental procedure ..................................................................................19

Nomeclature ................................................................................................................................................21

Literatur:.....................................................................................................................................................21



1. Introduction The liquid-liquid-extraction is the separation of a solution with an additional liquid component (solvent), which forms an miscibility-gap with the origin solution and has a sufficient solubility for the component to be separated.

This separation process can be an alternative or a supplement to distillation processes if in the mixture to be separated :

a) the components boiling points are close together, so that a rectification is only possible with numerous stages and a high reflux-ratio, which causes high energy costs,

b) some components form azeotropes while evaporating, so that a distillation process can only be per-formed by costly methods like two-pressure distillation or azeotropic or extractive rectification,

c) the concentration of the component to be separated is low and the boiling point is the highest in the mixture, so that the main components have to be distilled with high energy consumption,

d) temperature sensitive components are present, which can be extracted by a suitable solvent, from which they can be separated by distillation under mild conditions or by other processes like crystallization.

Usually the solvent has to be used in a closed cycle. The solvent recycling mostly is realized by distil-lation, in some cases by an additional extraction, and usually requires much expense. Whether a separation sequence using an extraction step is recommended can only be decided by evaluating the cost effective-ness of the complete process.

In spite of its significant expense, the liquid-liquid-extraction can be more economic than a direct recti-fication. An example is the concentrating of aqueous acetic acid solutions below 60 wt-%. The direct distil-lation is problematic and its technical application is additionally complicated due to by-products from the acetic acid synthesis. A suitable solvent is e.g. n-butyl acetate, as used in this experiment. Despite the fact that acetic acid is less soluble in such solvents than in water, and consequently large quantities of the solvent have to be used and regenerated by rectification, this extraction process is economically inter-esting.

In order to elucidate the liquid-liquid extraction the above mentioned industrial system is used in this experiment. But the direction of the extraction is inversed compared with the original process, i.e. acetic acid is extracted from n-butyl acetate by water. Therefore, the principle of the process can be demon-strated using smaller amounts of chemicals and easier analyzable concentrations.

The first aim of this experiment is learning to visualize ternary mixtures in triangular diagrams an the possibilities to make balances in such a diagram. Using some experiments the measurement and the possible methods are shown. Using the achieved abilities a four stage countercurrent extraction process will be developed by graphical methods and performed in a laboratory setup. Finally the theoretical and experimental data are compared and the efficiency of the laboratory setup will be determined. Please inform yourself in literature about technical extractors and their special properties and applications (overview in [1], more detailed in [2 - 4]).



2. Fundamentals 2.1 The Liquid-Liquid Extraction

Input Mixture F Extract E

Raffinate R Solvent L

Extraction Stage

Fig. 1 Schematic representation of an extraction-stage

The solution to be separated (input mixture or feed F), containing the carrier component T and the sub-stance to be separated C (extracted component), is contacted with a defined quantity of the solvent L. To improve the contact, the interfacial surface has to be enlarged, e.g. by dispersion. Because the ternary system T, L, and C forms a miscibility gap – which includes the mixture of feed and solvent if enough solvent is added – two ternary immiscible phases are formed. Between these two phases a mass transfer occurs: the substance C to be extracted is distributed between the carrier T and the solvent L. The extraction substance C leaves the carrier T until the phase equilibrium between the two phases is reached. After settling the two phases, the raffinate R, which was formed by the input mixture, is removed. The raffinate contains in the main the carrier T as well as a residue of the extraction substance C and solved solvent L. The extract-phase E, formed by the solvent L contains in the main the solvent L, the extracted substance C and solved carrier T. The predefiniton of carrier T and raffinate R respective solvent L and extract E is given by the process guidelines according to figure 1 and not perforce by the substances. As mentioned in the introduction, for the same substances different processes can applied, by the exchange of carrier and solvent.

The schematic pictured extraction stage in Fig.1 is formed by a mixer-settler combination (Fig.2). The mixer performs – usually mechanically e.g. by stirring – the good liquid-liquid contact of feed F and solvent L. The separation of the equilibrium phases raffinate R and extract E is performed in the settler.

Feed F Solvent L Raffinate R

phase interface

Extract E

Mixer Settler

Abb. 2: Structure of a Mixer-Settler Unit Each mixing and settling of the phases is called extraction-stage. The extraction process is formed by one or more sequenced stages. Mostly recommend in technical applications are multiple stage processes, because, supposed the same extraction effect, the solvent quantity can be lowered. The optimal conditions can be determined mathematically, presumed the knowledge of the phase-equilibrium, as well as graphically (As performed in this experiment). Extraction experiments are only required for not clearly defined solvent- or feed mixtures



The extraction can be divided into three characteristic units

• Contacting the input mixture and the solvent

The distribution equilibrium is reached faster by intensive mixing of both liquid phases. The mixing time and the quality of mixing specifies which degree of phase equilibrium can be reached. By enabling a very fine distribution, the equilibrium can be reached immediately using special mixers.

• Phase separation in layers

After leaving the mixer, the two liquids, the two liquids form two layers within the setller unit. For the process efficiency, this process is as important as reaching the equilibrium. Due to the properties of the mixture, e.g. a low density-difference and/or surface-tension between the phases, or too intensive mixing, difficulties in phase separation like forming of emulsions can be caused.

• Removing and recycling the solvent

The extraction is nearly always connected to other separation-techniques, e.g. distillation, to recycle the solvent. Often the solvent recycle is as important as the extraction itself.

An apparatus enabling the equilibrium between both phases, is defined as one theoretical extraction stage *). The convergence to the equilibrium is a degree for the efficiency of the apparatus. The efficiency, which indicates the difference between theoretical and practical (…), can often only be determined empiric.

The following chapter describes the graphical representation of extractive processes in different digrams for the ternary system. 2.2 The triangular diagram and its properties

Abb. 3: Concentrations for tenary mixtures in a triangular diagram (e.g. molar fraction)

Abb. 4: mixing rules and law of lever in a triangular diagram.

*) Analogue to other thermal separation processes, as rectification and absorption, there are several processes where

a continuous mass transfer occurs without strict separation of mixing and settling.( e.g. packing columns.) In these a part of the packing is defined which is equal to one equilibrium stage. It is a height-equivalent for a theoretical stage.



Triangular diagrams are advantageous for the graphical presentation of phase equilibria of liquid ternary systems at a constant temperature and pressure. We will use equilateral triangles due to the clear shape. Rectangular triangles are often used in literature (e.g.[5]), which does not change the coherence.

The pure components T, L and C are represented by the corners (Fig. 3). The composition of the three binary mixtures TL, LC and TC can be seen on the triangles sides. Every ternary mixture of T, L and C is represented by a point inside the triangle.

In the triangular diagram, concentrations are used according to the following term *)

x i

i

=∑ 11

bzw. x i

i

=∑ 1001

% (1)

e.g. molar fraction (Fig 3) or mole percent as well as mass fraction and mass percent, used in the following experiment**). Independent of a one or two-phase system, the concentration has to be valid for the total amount of the mixture (an concentration using mol/l would be pointless for a biphasic system).

The following rules are valid in a triangular diagram (Fig. 4):

1. According to the second intercepting theorem, every line starting in a corner divides the opposite side and all parallel lines in the same ratio.

Adding a third component C to a mixture M (containing L and C in a ratio of 3/7), all possible mixture are represented by the line TM, because the ratio L:C=3:7 does not change by adding T.

2. Using molar fraction as concentration, the molar ratio of the added component C to the moles of the former mixture M in the mixing point N is given by the ratio of the distances between the points:

MoleMole

MoleMole Mole

TM

TL C

MNTN

= + = (2)

This term can be converted into the lever law:

( ) ( ) ( ) ( )Mole MoleT TN M MN⋅ = ⋅ (3)

3. According to the above mentioned rules, the possible compositions of the new mixture R, formed out of two ternary mixtures F and S, are represented by the line FS. The lever law is valid as well. Figure out, that the lever law is the overall and partial mass balance (F and S are mixed in the ratio 1:2)

2.3 Binodal curves and tie lines

Only systems with at least three components and a miscibility gap can be used for extractions. Very important is the line between the liquid on-phase and two-phase zone.

Fig.5 shows schematically a ternary system, where C can be solved in T and L in every ratio and T and L are only partly soluble (miscibility gap).

*) By fixing the sum, the ternary system is determined by knowledge of two concentrations and can be displayed in a two-dimensional diagram.

**) Volume fractions or volume% are usually inapplicable, because volumes in a mixture can not be added by using equation (1) (e.g. water ethanol)



Abb. 5 Phase-equilibrium in a triangular diagram.

The distance in this triangular diagram represents the solubility of L in T. LY is the solubility of t in L.

The curve X-P-Y is the satured-isothermal at a constant pressure. It is also called binode or bimodal-curve. It is the frontier between the homogeneous one-phase area and heterogeneous two-phase-area. The temperature has a strong influence on this curve (in common the miscibility-gap shrinks at higher temperatures), while the pressure has nearly no influence.

Every point on the binodal-curve has another corresponding point on the binodal-curve. These two points represent the phase-equilibrium. The line betweens these two equilibrium-points are called tie lines.

All mixtures, which can be shown as a point in the miscibility-gap (e.g. K), desintegrate along a tie line into two ternary phases. The composition of the two phases in equilibrium is given by the endpoints of the tie lines on the binodal-curve. With T as carrier and L as solvent is, according to the definition, in Fig. 5 R the raffinate and e the extract (The mixture K disintegrates into the raffinate R1 and the extract E1).

The quantities of both equilibrium-phases, which are also called conjugated phases, is calculated by the law of lever:

KEceDis

KR

QuantityR

E

tan

DistanceQuantitity

Qunatity-Raffinate

Quantitity-Extract == (for the tie line K) (4)

The component C is distributed to both liquid phases. The tie lines shrink while the concentration of C increases. The composition of the equilibrium phases converge, till they get identic. They melt in the point P, the “critical point” which divides the extract-side from the raffinate-side. In systems with only one miscibility-gap, outside this gap, is only one homogeneous phase. In more complex systems, multiple miscibility-gaps can occur [5].

As shown in Fig. 5, the tie lines are usually not parallel. The slope can increase and decrease drastically, when the concentration of C changes. Generally the critical point P is not the maximum of the bimodal-curve nor is the maximum of the binodal curve in the middle of the diagram. If the slope of the tie lines change their orientation (e.g. from positive top negative) at least one horizontal tie line occurs (solutrope system). That means the concentration of C in extract and raffinate is equal. Please do realize, that the consequences are not the same like an aceotropic point in the distillation.

Generally it is more economic, if the component to extract is better soluble in the extract-phase than in the raffinate phase (corresponding to Fig. 5), but it is not the basic requirement, because the effect can be compensated by higher amounts of solvent.



2.4 Procedures for measurement and interpolation and model-equations for liquid-liquid-equilibria

For the visualisation of phase-equilibria, it is only necessary to determine a couple of tie lines and fit the binodal-curve to the endpoints of the tie lines.

To gain the maximum redundancy for the measurement results, it is necessary to use all achievable information:

• Composition of the origin mixture K • Composition of extract- and raffinate-phase via exact analysis of, if possible, all components • Amount of extract- and raffinate-phase, to control the balance by the law of lever

If not all of this possibilities to measure are available – as in this experiment – one can use less complex methods, which use only some of the a.m. data. An additional method for the determination of points on the binodal-curve is the “turbidity-titration”, which directly uses the transition from the homogeneous to the heterogeneous and the reverse effect.

It is necessary to be able to determine every point of a binodal-curve and the associated tie line for the design of an extractive process. All measurement results are defective, even if all above mentioned possibilities for the determination and control of measurements are used. If one has determined sufficient tie lines, the first idea is linear interpolation between the tie lines to determine the values in between. One would presume, without knowledge, that there is al linear connection between the tie lines.

Abb. 6 Interpolation procedure using

auxiliary lines

It is more suggestive to view all data as an unity and evaluate all together for the binodal-curve and the slope of the tie lines.

For the graphical interpolation of the tie lines we use the “auxiliary line” following the methods in Fig. 6.

The more effective Method uses a second triangular diagram T-L-G. Every point P-X-Y-Z of the auxiliary line

is constructed by the end-points Ri and Ei of a tie line, using parallel lines to the triangle T-L-G (e.g. point Y using R2 and E2). The auxiliary line runs from point Z, which represents the solubility of T in L, to the critical point P. Without a second triangular diagram, another auxiliary line P-U-V-W can be constructed, using parallel lines to T-L and T-C. Due to lower cutting angles this method is less precise.

Using the auxiliary lines one is able to construct every conjugated point, e.g. E3 with X or U leads to R3

The main advantage of this graphic method is the ability to construct fit-curves for the binodal-curve and auxiliary line and obviously erroneous values can be determined.



Besides this graphical method, mainly model-equations are used today, especially for the application in computer programs. These are based prevailing on model equations for vapour-liquid-equilibria (see experiment TC1) like the NRTL-Equation (Non Random Two-Liquid) and UNIQUAC-Equation (Universal Quasi-Chemical) [5]. A Liquid-mixture with a miscibility-gap is an example for a major deviation from the ideal behaviour of a liquid-mixture, where no component interaction occurs. The named equations are able to describe the vapour-liquid-equilibria of non-ideal multi-component mixtures. The connection to liquid-liquid-equilibria is given by the fact, that in equilibrium the vapour-phase has to be the same.

Aided by these equations and a parameter-fit, it is possible to reproduce the measurement and control the data. Data for liquid-liquid-equilibria and parameters are available in [5]. An intensive field of research are group-contribution model, which allow data prediction out of the molecular structure without extensive measurement. One method is the UNIFAC-method (Universal Functional Group Activity Coefficients) which is a derivative of the UNIQUAC-method [6]. 2.5 The equilibrium diagram

In addition to the ternary-maps, distributions-diagrams are used, which are based on rectangular coordinates (analogous to the McCabe-Thiele-diagram in the distillation). These diagrams only represent the tie lines, which mean the distribution of the concentration for one component between two heterogeneous phases. Solubilities, which are represented by a triangular diagram, can’t be represented by a equilibrium diagram. Even though these diagrams are often sufficient and easier to handle. A special use for equilibrium diagrams is the comparison of different solvents.

The concentrations of the substance to be extracted in both phases (y=extract, x=raffinate) are plotted against each other. The construction out of the triangular diagram and the connection between these diagrams is shown in fig. 7.

Abb. 7 Connection between triangular diagrams and the

equilibrium diagram

Via the transmission of the solubilities of a ternary system with rare measurements the tie lines can be well interpolated. The determination of an unknown tie line through the point M has to be tried out by iterative testing of values.



2.6 Extraction processes The following extraction-processes are industrially used:

a) the one-stage extraction-process b) the multiple-stage extraction-processes

• cross-stream-extraction • countercurrent-extraction • countercurrent-extraction with reflux

2.6.1 The one-stage extraction

Only rarely used in technical processes, the main application of a one-stage extraction in the laboratory scale (e.g. separating funnel).

The schema of this process was given in fig. 1: An amount of the mixture F (contributed by the carrier T and the substance to be extracted C) is contacted and mixed with a solvent L, to quickly obtain equlibrium. After both phases have settled, the extract and the raffinate are separated. The one-stage extraction can be performed in batch and continuously. 2.6.2 The multiple-stage extraction-processes 2.6.2.1 The cross-stream extraction

Abb. 8 Cross-stream extraction of feed/raffinate and solvent It can be more effective if the total amount of solvent is split and the extraction is performed stage-wise, which means several mixer-settler-units are connected consecutively and each stage is fed with fresh solvent.. The raffinate of one unit is the feed of the next unit. Fig. 8 shows a multiple-stage cross-stream extraction. It is possible to perform the cross-steam extraction in batch or continuously. In the continuous process, the feed flows through the extraction units and is contacted with fresh solvent in every unit. After the last extraction unit, the raffinate leaves the process. Every unit produces an extract, in which the concentration of the component to be extracted lowers from unit to unit. In the analogical batch process, the Feed F is contacted several times with fresh solvent, after removing the extract at a time.



A difference between the batch an the continuous process is the mass-balance driven concentration-change, which is a function of time in the batch-process and a function of locus in the continuous process. 2.6.2.2 The countercurrent extraction

Abb. 9 Countercurrent extraction

The multiple-stage countercurrent-extraction according to fig. 9 is technically realised in continuous processes. The feed and the solvent is fed to the process on the opposite ends and flow continuously through the process: The raffinate of one stage is the feed of the next and the extract of one stage is the solvent of the previous one.

The countercurrent extraction is the most common extraction process in industrial use. Due to the high driving force, caused by high concentration differences, the equilibrium is reached fast. The countercurrent principle is also used in other process techniques, e.g. heat exchanger. The raffinate is contacted with fresh solvent, which causes an optimal depletion. The extract reaches the highest possible concentration of the substance to be extracted, by being contacted with the feed. In general, the best results with the lowest amount of solvent can be reached with countercurrent extractions.

A typical application for a product-reflux is the distillation. By enriching the liquid phase with the product, the product-concentration in the vapour phase increases an the product-purity increases. Some application make it necessary to use a reflux in the extraction too, due to the slope of the tie lines , the miscibility-gap increases or the risk of emulsions decreases.



2.7 The representation of a a countercurrent extraction in a triangular diagram Fig. 9 shows a multiple-stage countercurrent process, containing 4 mixer-settler units. A countercurrent extraction cannot be constructed stage by stage in a triangular diagram like a cross-stream-extraction, because the raffinate concentration at a time is currently unknown.

A mass-balance around the first, the first an the second and around the first and the n-th stage leads to, according to fig. 10:

Abb. 10 Mass balance for the countercurrent extraction 1. stage m F + m E 2 = m R 1 + m E 1 m R 1 – m E 2 = m F – m E 1 = Q (5)

1. to 2. stage m R 2 – m E 3 = m F – m E 1 = Q (6) • • •

1. to n-th stage m R n – m L = m F – m E 1 = Q (7)

According to these equations one can define for every stage a constant mass-difference-stream Q, which is equal to the difference between the feed and the extract of the first stage.

Abb. 11 Graphical construction for the determination of the number of stages in countercurrent extraction Every balance-equation (5) to (7) is, according to the mixture law, a line in the triangular diagram. For every stage I, the line cuts the point for the leaving raffinate R i and the incoming extract E i + 1 (for the n-th stage, the solvent L). The lines for all n stages meet also in one fixed point Q, which is therefore called pole or working-point (fig. 11, for process feed, the line Q-F-E1 has to be added)



Usually the position of the pole Q is outside the diagram (right or left, depending on the position of F end E1) and represents only a fictional composition, because negative amounts and negative concentration don’t exist. One can’t have a real association of this composition due to it is only a fictional mixture, which is mathematically founded in the equations mentioned above.

The position of Q can be determined if the concentrations are known or demanded: Die Lage des Pols Q läßt sich einfach festlegen, wenn die Konzentrationen bekannt bzw. gefordert sind:

• the feed F in the first stage • the leaving extract in the first stage • the leaving raffinate in n-th stage, determined by the problem

• the solvent incoming to the last stage (usually pure solvent)

The procedure according to fig. 11 is following:

Beginning with any end of the process, e.g. stage 1, where the feed flows into the plant and the final extract leaves the plant, one draws a line through the points F and E1 as well as through Rn and L. The two line meet in the point Q. Aided by the tie line E1-R1, the composition of the raffinate leaving the first stage can be determined, and the line R1 – Q can be drawn, according to eq. (5). With this line, the concentration of the extract E2 can be determined, which is leaving the second stage and entering the first stage. The corresponding raffinate R2, leaving the second stage is given be the tie line through E1.

This construction uses on one hand the mass balance of the components, which flow in the pipes outside the extraction-stages – visualized by the lines through the pole Q – and on the other hand the phase equilibrium - visualized by the tie lines, which can be drawn by using the auxiliary line.

One will proceed, until the wanted raffinate concentration is reached or exceeded. The number of stages (phase-equlibria), which are needed to reach this aim is equal to the number of theoretical extraction-stages. It can make sense, to estimate fractional stages, because the practical needed number of stages is higher than the theoretical one (practical number of stages = theoretical number of stages / efficiency, view chapter 2.10) 2.8 Stage construction with different starting points

Abb. 12 Countercurrent extraction: Connection between solvent/feed ratio and the end-concentration

of extract and raffinate

Due to technical and/or economic reasons three of the following parameters can be specified, the two other follow consequential:

• feed-concentration (usually fixed) • raffinate concentration • extract concentration • solvent-feed ratio • number of stages (including efficiency)

The mixing-point M, calculated by the amounts of solvent and feed, cut the connection-line FL according to the law of levers:

FM

LM

amount=

L

Famount (8)



By converting the equation (7) m m m mR n L F E− = − 1 (7)

The result is m m m m MF L E R n+ = + =1 (9)

Which means, also a line through E1 and Rn cuts the point M. If the composition of E1 and Rn are given, the solvent-feed ration L/F can be calculated. On the other hand, if L/F and Rn or E1 is given, one can construct the point M and with E1 or Rn the opposite one ba using the bimodal curve.

The further estimation of the pole Q and the number of stages n follows the above mentioned rules.

If the number of stages shall be fixed, the procedure is the same, but the solution has to be found iterativly. Using a fixed feed, raffinate Rn and extract E1, with a fixed number of stages, the solvent-feed ratio L/F has to be varied. First a ratio L/F is used, to make the construction according to fig. 12. Thereafter, e.g. starting with E1, the graphical construction has to be made. If the end-concentration is higher (lower) than demanded, the ratio L/F is too low (high) and has to be varied till the demanded concentration is reached. Within this iteration, the second composition E1 and Rn as well as the pole Q will change. 2.9 Transmission of the stage construction into the equilibrium diagram

On the ordinate of the in chapter 2.5 presented equilibrium diagram, the concentration y of the substance to be extracted is plotted. The abscissa represents the concentration x in raffinate. The resulting curve is the equilibrium between the two phases.

Abb. 13 Transmission of the stage-construction from a triangular diagram into a equilibrium diagram For the construction of the working-line *) a cross-section in the schematic extraction-process (shown in fig 13) is contemplated. In this cross-section the streams are, e.g. the feed F with the concentration xF and the extract of the first stage E1 with the concentration yE1. Both concentrations are the end-points of the working-line in the equilibrium diagram. This procedure has to be made for all cross-sections.

*) The working line for an extraction does not have to be a straight line, because the mass flow of extract and raffinate between the stages may differ.



The other end-point of the working-line is fixed by the concentration yL of the solvent (using pure solvent,

yL=0) and the concentration xRn of the leaving raffinate.

Beginning with the feed concentration, a stage-construction is drawn between the distribution curve and the working-line. The stage construction in the equilibrium diagram has to have the same number of stages as in the triangular diagram. 2.10 The practical extraction-stage and the effiecency

With the stage-construction of fig. 13, the only the theoretical number of stages can be determined, because the assumption was equilibrium between the two phases. In a technical extraction-apparatus, which means a practical extraction-stage, the equilibrium cannot be reached in total. To describe the efficiency of a practical extraction-stage, the efficiency-factor n is defined as follows (related to the substance to be extracted in the extraction-phase)::

1

1

1

enrichment possibel ltheoretica

enrichment practical≤

+−′+−

==iE

yiE

y

iEy

iEy

η (10)

y´E i : theoretical equlibrium concentration of C, which

is maximal achievable in stage i

yE I : practical (measured)

concentration of C in the extract of the stage i

yE i + 1 : practical (measured) concentration of C in the extract of the stage i+1, the feed of stage i

Abb. 14 Determination of stage efficiency

The efficiency can be presented best in the equilibrium diagram (fig.14). According to eq.(10), the stage-efficiency of stage i is given by the ratio of the line i-i’’, which is the practical reached concentration-difference, and the line i’-i’’, which is the theoretical concentration-difference in equilibrium. On the other hand, the stage construction can be made for technical extractions by using a stage height for the extraction which uses the efficiency-factor.

The equilibrium cannot be reached, due to these main reasons: • The equilibrium is not reached in the mixer, because:

o The mixing and within this the interfacial area is not sufficient o The contact time is too low (related to the interfacial area)

• The separation is not sufficient, parts of the other phase reach the next extraction stage

The efficiency is not only caused by the construction of the apparatus, but also by the substances and the process-parameters, especially the mass flow



3. Experiment description

3.1 Physical properties

Carrier T n-butyl-acetate (acetic acid – butyl ester, „BuAc“), molar weight 116,16 kg/kMol Etractant C Acetic acid („HAc“), molar weight 60,05 kg/kMol Solevent L Water, molar weight 18,02 kg/kMol

All substances are thought as pure.

The following data for the liquid-liquid equilibrium at 25°C are taken from literature [5], including the parameters for the model-equations (chapter 2.4). The model equations are based on the interaction between the molecules and use molar concentrations (molar fraction or mol%). Aided by the easy analysis methods (chapter 4.6) the molar fractions are nor directly achievable. This is why this experiment uses weight fraction (weight%) and all data from [5] are calculated to weight%. This doesn’t change the principle of methods mentioned in chapter 2 *). The following tabular contains the end-points of 15 tie lines as well as the critical point (the assignment extract and raffinate is an assumption for this experiment)

Tie line Weight% in the raffinate Weight% in the extract No. BuAc HAc Water BuAc HAc Water

1 97,1 1,3 1,6 0,8 3,7 95,5 2 94,8 3,5 1,7 1,0 6,7 92,3 3 93,0 5,0 2,0 1,0 10,1 88,9 4 90,8 6,4 2,8 1,2 13,2 85,6 5 88,0 8,9 3,1 1,3 16,1 82,6 6 85,1 10,9 4,0 1,6 19,3 79,1 7 81,4 13,6 5,0 1,7 21,5 76,8 8 79,8 14,6 5,6 1,9 25,3 72,8 9 77,0 16,7 6,3 2,3 27,8 69,9 10 72,1 20,4 7,5 2,9 30,7 66,4 11 70,2 21,4 8,4 3,9 32,8 63,3 12 64,8 25,2 10,0 4,7 35,3 60,0 13 60,5 27,8 11,7 6,1 37,6 56,3 14 54,2 31,4 14,4 8,9 39,4 51,7 15 45,5 35,8 18,7 12,8 42,1 45,1

The coordinates for the critical point were calculated by customized model equations

P 26,1 43,3 30,6

Tabelle 1 Liquid-liquid phase-equilibrium at 25°C for the system Acetic-acis – n-butyl-acetate – water [5]

*) Due to the high difference between the molar masses of n-butyl-acetate and water the numerical values of the equilibrium data in mass% and mole% is high. The slope of the tie lines is also not the same. In molar concentration, the solubility of acetic-acid in n-butyl-acetate is higher than in water, in mass concentration we have the reverse effect. In total the choice of the concentration unit has no effect on the plant design.



3.2 Methods for the estimation of liquid-liquid phase-equilibria

As mentioned in chapter 2.4 the exact weighting in of a mixture in the homogeneous area, the exact separation of both phases and the determination of all concentrations are the best condition to gain reliable data. In this experiment some easier methods shall be presented and experimental proven 3.2.1 Turbidity-titration for estimation of the binodal curve

By drop-wise addition of a substance to a known mixture, the binodal curve can be touched, which causes a change from the homogeneous to the heterogeneous area and the solution turbids, or the reverse effect. By this easy method and the evaluation using the law of lever (fig. 4), discrete points on the binodal curve can be determined. This method delivers directly the complete composition in mol% or weight% of a measured point.

For the determination of the complete binodal-curve it is necessary to chose mixtures which cause equidistant points on the binodal-curve. One point should touch the maximum of the curve. 3.2.2 Investigation of both phases of a heterogeneous mixture

HAc C

Wasser L

mER

E

BuAc T

MmR

Konode

Abb. 15 Using the law of lever to fit the data for the mixture M to the

binodal curve.

Starting with a known heterogeneous mixture (point M in fig. 15) it is known that both phases E and R lie on the same line with M in a triangular diagram. The mass of both phases can be determined by separating and weighing both phases or by quantifying the volume and the density of both phases. Due to the lever-law-equations (2) and (3) the mass ratio is known and using this the ratio of the distances E-M to M-R is available.

For the determination of the precise position in the triangular diagram, additional informations are needed.

3.2.2.1 Fitting the binodal-curve

If the binodal-curve is known, e.g. by turbidity-titration, one can use the fact that E and R are part of the binodal-curve. By trying out, one can fit the tie line through the point M by using a ruler, being aware to fit the phase-mass ratio. 3.2.2.2 Analyses of components in the phases

As an easy method, the titration of acetic acid with 0,1 n sodium-hydroxid-solution**) is available. Two horizontal lines can be drawn by using this method. The tie line can be fixed with this method as well by trying out to fit the points R and E to the concentration lines of C.

*) Using this way, only the ratio between the masses not the moles is available. The results can only be displayed in a triangular diagram using mass%.

**) The titration uses phenolphtalein as indicator for a light alkaline switsching point as needed for acetic acid. The saponification of n-butyl-acetate is negligible.



3.3 Laboratory plant for the contiuous four Stage countercurrent extraction

Abb. 16 Flow-sheet of the continuous countercurrent four-stage extraction

The schematic flowsheet of the plant for the continuous working countercurrent extraction is shown in fig. 16. The necessity for those plants is given by the investigation for new developments and to get information about the efficiency if mixer-settler-units which cannot be found by theoretically knowledge of the equilibrium data. The feed and the solvent is fed continuously into the plant by two metering-pumps out of two storage tanks

Abb. 17 Mixer-Settler-Unit of the laboratory apparatus.

Every extraction-stage is a combination of one mixer and one settler unit, shown in fig. 17 (the apparatus can differ). In its function, the mixer is analogue to a rotary-pump. Extract and raffinate are fed separately, climb in the cavernous shaft and are fling out into the mixing room through the cavernous stirrer. Here an intensive mixing occurs. After the mixer the mixture flows into the settler and is separated under influence of the gravity. The lighter phase leaves through the spillover and the heavy phase flows through holes into the outer pipe of the settler. The heavy phase leaves the settler through the concentric pipe in the middle of the outer pipe.



4. Task formulation and experimental procedure The substances acetic-acid (corrosive) and n-butyl-acetate (smelly) have to

be handled very carefully!

The safety regulations have to be abided strictly!

We allude to your duty to wear safety glasses

The feed has to be mixed in the flue and may only be transport in unbreakable tank!

Mixtures containing n-butyl-acetate may not attain the sewage (not even the titrated solutions)

The raffinate may not be put into the waste-container. It has to be straged in the provided tank in the flue!

1. Enter the data (chapter 3.1)into the triangular diagram provided by the assistant

• Draw the binodal-curve, the tie lines and the auxiliary line as compensation-curves. This work is a prerequisite for a successfull experiment and should be done very precise

2. Determine the solubilties of n-butyl-acetate and water in each other by turbidity-titration • Use app. 100 g n-butyl-acetate (exact weight) and titrate with water (density 1kg/l

• Use app. 100 g water (exact weight) and titrate with n-butyl-acetate (determination of the density aided by the Mohr-Westphal-Scale*)

• Calculate in weight% and enter the data into the triangular diagram. Do discuss the results and deviation from the literature-data

3. Determine two tie lines be phase separating as mentioned in chapter 3.2.2 • Make two solution with app 100g in a concentration given by the assistant (take care to weigh

exact) in a closable measuring cylinder and realize equilibrium by shaking. After settling determine the volume of both phases.

• Fit to the binodal curve o Determine the density of the phases by use of the mohr-westphal scale o Determine the result according to chapter 3.2.2.1 and enter it into the triangular diagram.

Discuss the results and the deviation from the literature data. • Analysis of acetic-acid in both phases

o Titrate the exact scaled samples with 0.1 caustic soda **) o Determine the result according to chapter 3.2.2.2, enter it into the triangular diagram and

discuss the reults and the possible deviation from the literature data

*) The manual for the mohr-westphal-scale can be found in the laboratory.

**) Estimate the sample-mass, so you need roundabout 10ml 0,1n NaOH.



4. Determine the concentration of acetic-acid in the raffinate left from the last experimental course with 0.1 n caustic soda

• Determine the complete composition by using the binodal-curve • Determine the mass and concentration of acetic-acid in the left feed from the last experimental

course with caustic soda.

5. Using the old feed and raffinate and the analysis results of point 4, mix app. 3kg new feed mixture by adding acetic-acid and n-butyl-acetate. The feed ratio should be 40 weight% acetic-acid and 60 weight% n-butyl-acetate. Control the acetic-acid concentration by titration and fill in the mixture into the feed-tank. • Draw the feed-point into the triangular diagram.

6. Empty the feed-tank for the solvent water and fill in an exact amount of water. Empty the raffinate

and extract waste-tanks.

7. Make a stage-construction for four theoretical Stages according to chapter 2.8 to determine the neccessary solvent/feed-ratio and the extract end-concentration to reach the raffinate end-concentration given by the assistant. (Feed-mixture see point 5, a starting point for the L/F-ratio is given by the assistant)

8. Start the plant after consulting th technician or the assistant with the above determined solvent/feed- ratio. • At first check the metering pumps*) • Note the starting time and all actions

9. After app. 2 hours the first raffinate sample has to be taken (raffinate 4), using an injection syringe with attached PTFE-tube. 2-3 g of this sample are scaled into an Erlenmeyer flask (extactness 0.001 g) diluted with water and titrated with 0,1n caustic soda against phenolphtalein. The next samples are taken in 20 min stages.

If the deviation of two samples is less than 5% relative, the equilibrium is reached.

10. Shut down the plant to ensure the concentration of the following samples don’t change (please note the time.

11. The real L/F ratio which is neccessary for the triangular diagram is calculated by the used masses. Therefore , the remaining feed and solvent masses are determined and the substance are refilled into the storage tanks.

12. Determine the total amounts of raffinate and extract by using a scale.

The raffinate has to be stored in the raffinate storage-tank in the flue for the next experimental course. The raffinate is filled into the waste.

*) Pay attention to all pipes are free from air. If air gets into the pumps, the perfusion will break sown.



13. Take samples of extract and raffinate out of every settler by using a dry syringe with attached PTFE-tube. These 8 samples are titrated to determine the concentration of acetic-acid. Every concentration has to be determined twice. If the deviation is higher than 5% relative to the mean value, the titration has to be done again.

14. Make a stage-construction with the real L/F ratio (point 11) • Enter the measured raffinate and extract-concentrations into the triangular diagram. • Make a balance around the continuous countercurrent extraction using all measured mass-streams

and analysis results (L, F, R4, E1) The theoretical and practical ratio of L/F and R4/E1 have to be compared.

These results (point 14) have to be presented to the assistent on the end of the experimental course

15. Draw a equilibrium diagram and construct the equilibrium-curve • Transmit the theoretical Stage-construction as well as the measured data and sketch the resulting

working-lines. • Calculate the efficency-factors of all extraction-stages

16. For the experiment-discussion, all scetched diagrams have to be discussed with qualitative information for the curves. Following, the calculated efficiency-factor have to be discussed.

Nomeclature

BuAc = n-butyl-acetate HAc = acetic-acid

C = substance to be extracted E = extract F = feed L = solvent M = mixing point

P = critical point R = raffinate T = carrier substance

m = mass [kg , g] n = number of extraction-stages

η = efficiency-factor ρ = density [g / ml]

Literatur:

[1] Ullmann´s Encyclopedia of Industrial Chemistry, 5th Edition, Volume B3 „Unit operations II“, Chapter „Liquid-liquid extraction“, VCH Verlagsgesellschaft mbH, Weinheim, 1988

Ullmanns Encyklopädie der technischen Chemie, 4. Auflage, Band 2 „Verfahrenstechnik I, Grundoperationen“, Kapitel „Flüssig-flüssig-Extraktion“, Verlag Chemie, Weinheim, 1972

[2] Gmehling, J.; Brehm, A.: „Lehrbuch der Technischen Chemie“, Band 2 „Grundoperationen“, Georg Thieme Verlag, Stuttgart, 1996

[3] Sattler, K.: „Thermische Trennverfahren“, VCH Verlagsgesellschaft mbH, Weinheim, 1995

[4] Schlünder, E., Thurner, F., „Destillation, Absorption, Extraktion“, Georg Thieme Verlag, Stuttgart, 1986

[5] Sørensen, J. M.; Arlt, W.: „Liquid-liquid equilibrium data collection“, Chemistry Data Series, Vol. V, DECHEMA Frankfurt am Main, ab 1979

[6] Lohmann, J.; Joh, R.; Nienhaus, B,; Gmehling, J.: „Verbesserung der Vorhersagequalität und Erwei-terung der Anwendungsgebiete der Gruppenbeitragsmethode Modified UNIFAC (Dortmund)“,

Chem. Ing. Tech. 70 (1998) 138-142

[7] Welzenbacher, U., „Neue Datenblätter für gefährliche Arbeitsstoffe nach der Gefahrstoffverordnung“, WEKA Verlag, Augsburg 1994

Documents

Experiment TC 31